Update gui-faq-da.md & messages.da.xlf

[haarbo]

Purpose

This PR...

Context

Fixes #390 #391

Changes

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How to test this PR

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[matsduf]

@haarbo, I pushed a commit with some editorial updates of the XLF file:

• Some translations lacked final dot were the source has one. It was corrected.
• All entries were marked as new. I updated them to translated.

[matsduf]

@mattias-p, do you want to review?